The purpose of breathing is to provide a way for the body to receive fresh oxygen in exchange for the carbon dioxide and other waste gases that the cells of the body have produced. During this exchange of gases, between six and 10 liters of fresh air per minute is brought into the lungs.
Inhaled air fills the alveoli, which are only one-cell thick and are surrounded by capillaries that are also one-cell thick. Oxygen passes through the air-blood barrier quickly and into the blood in the capillaries. About 0.3 of a liter of oxygen are transferred from the alveoli to the blood each minute. In much the same way, the carbon dioxide passes from the blood into the alveoli and is then exhaled. About 0.3 of a liter of carbon dioxide flows across the walls of the capillaries and the alveoli to be exhaled each minute.
Blood loaded with fresh oxygen flows out of the lungs through the pulmonary veins and into the left side of the heart, which pumps the blood to the rest of the body. Oxygen-depleted, carbon dioxide-rich blood returns to the right side of the heart through two large veins, the superior vena cava and the inferior vena cava. Then the blood is pumped through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide.
During exercise, we can breathe in as much as 100 liters of air per minute. The rate at which oxygen enters the body is one way to measure how much energy the body is using.